This invention relates to a method for cleaning fuel gases such as those produced in the gasification of coal or in petroleum and natural gas refining processes. It may also have application in other chemical fields in which sulfur compounds or particulate material are to be removed from carbonaceous gas streams.
This process has particular application for cleaning fuel gases produced in the gasification of coal. Such gases contain particulate materials such as ash components of the coal and finely divided char particles in suspension. The gas also includes hydrogen sulfide derived from pyrite or other forms of sulfur in the coal. Other sulfur compounds, such as carbonyl sulfide and various organic sulfur compounds, also may be released on gasification.
Molten salts are advantageously used for cleaning fuel gases at temperatures near those at which the gases emerge from the gasifier to conserve sensible heat. The alkali metal and alkaline earth metal carbonates are well suited for this purpose as they react with hydrogen sulfide to form metal sulfide and thereby remove sulfur from the fuel gas. The metal sulfides in the salts can be regenerated to carbonates through reaction with a strip gas such as steam and carbon dioxide.
Various carbonate salt compositions can be used. The ternary eutectic mixture of, by mole proportion, 43.5% lithium carbonate, 31.5% sodium carbonate and 25% potassium carbonate is advantageous in that it has a low melting temperature of about 395.degree. C. Calcium carbonate can be dissolved up to about 20% in this eutectic or substituted for a portion of the lithium carbonate with some increase in melting point. Typical salt compositions including in mole proportion, 15 to 40% lithium carbonate, 25 to 40% sodium carbonate, 25 to 35% potassium carbonate, and 10 to 20% calcium carbonate have been suggested. Molten salt compositions in these ranges are not so volatile as to contribute significant impurities to the clean fuel gas. Such salts also tend to catalyze the hydrogen reduction of carbonyl sulfide to hydrogen sulfide but are otherwise fairly inert to fuel gas constituents.
A process for use of such a salt composition is presented in U.S. Pat. No. 3,919,390 to one of the present inventors and assigned to the United States Government. Other processes that include the use of molten salts for cleaning fuel gases are illustrated in U.S. Pat. No. 3,671,185 and U.S. Pat. No. 3,438,722.
U.S. Pat. No. 3,919,390 also discloses the use of a venturi scrubber for contacting fuel gas with molten salt. The venturi scrubber is a concurrent flow contactor. It provides about a single equilibrium extraction stage which is generally adequate for more than 90% removal of sulfur compounds. This removal efficiency can be achieved provided the salt is not loaded too heavily with the metal sulfides. However, the inventors have found that if the molten salt is maintained at a sufficiently low metal sulfide level for good removal of the sulfur compounds from the fuel gas, the concentration of hydrogen sulfide in the strip gas used to regenerate the salt also will be low. Low hydrogen sulfide concentrations, e.g. less than about 10% by volume in the discharge strip gas from the regenerator, are undesirable for efficient operation of a Claus process for the recovery of sulfur.
The use of multiple stages to obtain the desired concentrations of sulfur compounds will ordinarily require pumping devices for process fluid transfer. Unfortunately, molten salt mixtures of the type suggested are quite corrosive. Consequently the use of conventional rotary and reciprocating pumps with moving parts can cause process problems due to pump failure.
Therefore, in view of these disadvantages of the prior art systems, it is an object of the present invention to provide an improved process employing a molten, metal-carbonate salt for the removal of sulfur compounds and particulates from a fuel gas.
It is a further object to provide a process in which the hydrogen sulfide stripped from the fuel gas can be discharged at a sufficiently high concentration for use as an effective feed to a sulfur recovery process while still removing most of the sulfur from the fuel gas.
It is also an object to provide such a process with minimal use of mechanical pumping devices having moving parts for conveying the molten salt within the process.